Blender linear RGB to sRGB conversion
If you select default, the transform will be the sRGB OETF as applied via OCIO. If you select any of the other view transforms such as Filmic, it's an entirely different dynamic range and transfer function.

The discrepancies you are seeing may be a byproduct of the LUT approach, but I'm not sure.

Converting a scene referred EXR to a display referred PNG isn't quite as simple as using the sRGB OETF however, and YMMV depending on your needs.